Determining the frequency of asbestos use in automotive brakes from a fleet of on-road California vehicles

Our plain-English summary of Determining the frequency of asbestos use in automotive brakes from a fleet of on-road California vehicles, by J. De Vita, S. Wall, J. Wagner, et al (Environmental Science & Technology, 2012).

The study

• Authors: J. De Vita, S. Wall, J. Wagner, Z.M. Wang, L.E. Rao
• Journal: Environmental Science & Technology
• Published: 2012;46(3):1344-51
• Cited by: 3 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Asbestos is a known human carcinogen, and recent regulation in California limits asbestiform fibers in brakes to trace levels beginning in 2014, although there is no corresponding federal requirement. In order to gauge the current prevalence of asbestos use in automotive brake applications, the California Air Resources Board tested brake linings from 137 light- and medium-duty vehicles and 54 heavy-duty vehicles. Only about 3% of the light- and medium-duty vehicle brake linings contained chrysotile asbestos. All of those brake linings were drum-type shoes, which are generally being phased out. No asbestos was found in low mileage vehicles presumed to have their original stock linings from the vehicle manufacturer. Additionally, no asbestos was found in the heavy-duty vehicle brake shoe linings sampled. Given the small percentage of vehicle brake linings with asbestos observed, it appears that the prior federal ban that was subsequently overturned, in combination with a threat of litigation, has reduced asbestos use in brake linings. However, our study was limited in scope and without a national ban, the current and future prevalence of asbestos in brakes is uncertain, suggesting the need for continued monitoring of materials released as toxic air contaminants in normal braking operations.

In plain English

This study tested brake linings from vehicles on California roads to estimate how often asbestos was still being used in brakes. It examined linings from 137 light- and medium-duty vehicles randomly selected from state registration records in the Los Angeles area, and from 54 heavy-duty vehicles such as trucks and buses. Linings were screened under the microscope for asbestos. Four of the 137 light- and medium-duty vehicles (2.9%) had linings containing chrysotile asbestos, all in rear drum brakes. These linings averaged 27.5% asbestos by weight. No asbestos was found in any heavy-duty linings. The four vehicles were older and had higher mileage than the sample average. Their linings were judged likely to be aftermarket replacements, and three came from the same Canadian manufacturer. The authors concluded that the findings supported the view that asbestos use in U.S. brake linings was trending downward. They cautioned that the study was a snapshot in time, with small numbers and possibly biased samples.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Cross-sectional survey with laboratory analysis of brake lining samples from on-road vehicles
• Who was studied: 137 light- and medium-duty vehicles randomly selected from the California Department of Motor Vehicles registration database within about 50 miles of El Monte (Los Angeles area), model years about 1989 to 2006, sampled January 2007 to July 2008. Also 54 heavy-duty vehicles (line-haul tractors and trailers, city-delivery vehicles, motor homes, dump trucks, buses) from repair shops mainly in southern California. In total, 422 brake lining samples were analyzed.
• Exposure: Asbestos content of brake linings. Linings were screened for chrysotile asbestos by stereozoom and polarized light microscopy (NIOSH Method 9002, detection limit below 1% asbestos). Positive samples were ashed to measure weight percent asbestos.
• Main results:
  • 4 of 137 light- and medium-duty vehicles (2.9%) had brake linings containing asbestos.
  • In all four light- and medium-duty vehicles, the asbestos was identified as chrysotile.
  • All of the asbestos-containing linings were rear drum brakes.
  • No asbestos was found in any of the heavy-duty vehicle brake linings sampled (54 vehicles, 148 samples).
  • In the four asbestos-containing linings, the mean asbestos content per vehicle ranged from 25 to 30 weight percent, with an overall mean of 27.5 weight percent.
  • Cross-section subsamples contained significantly more asbestos than outer-surface subsamples (p = 0.023).
  • The four vehicles with asbestos linings had an average model year of 1994, compared with 1999 for the sample fleet.
  • The four vehicles with asbestos linings averaged 148,000 miles on the odometer, compared with 101,000 miles for the sample fleet.
  • All four vehicles were assumed to have had their brakes replaced at least once, indicating a high probability that the linings were aftermarket.
  • Three of the four asbestos-containing linings came from the same Canadian manufacturer. The fourth lining's manufacturer was unknown.
• The authors' conclusion: The authors concluded that finding asbestos in only 3% of light- and medium-duty vehicles and 0% of heavy-duty vehicles supported the contention of a downward trend in U.S. use of asbestos in brake linings.
• Limitations the authors state:
  • The study was a snapshot over a short period, so it cannot show past or future prevalence.
  • There were several possible sources of bias in the light- and medium-duty sample, such as over-representation of low-cost and high-mileage vehicles.
  • Sampling was in southern California; asbestos linings may be more common in northern California and more northern states.
  • The heavy-duty vehicles could not be sampled randomly.
  • The small number of heavy-duty vehicles (54) may explain why no asbestos was found in them.
  • Only four vehicles had asbestos linings, so the findings about them should not be extrapolated to a larger fleet.
  • The difference in asbestos content between lining locations was only a weak trend, largely because of the small sample size.

Topics

Medical terms in this study, explained in our Lexicon: Asbestiform, Brake Lining, Chrysotile.

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